Adaptive modulation has heretofore been known as one of techniques to improve a communication speed in a radio communication system. In the adaptive modulation, a modulation method using a high communication rate is employed when a propagation path has a good condition while a modulation method using a low communication rate is employed when the transmission path has a bad condition.
To achieve this, in a radio communication system using the adaptive modulation, a radio communication terminal measures communication quality of radio communication with a radio base station and notifies the radio base station of the measured communication quality. The radio base station selects the modulation method depending on the communication quality notified by the radio communication terminal.
In the meantime, an orthogonal frequency division multiple access (OFDMA) scheme has been known as one of multicarrier communication schemes using numerous subcarriers. In the OFDMA scheme, subchannels (hereinafter referred to as “communication channels”) each including multiple subcarriers are dynamically allocated to radio communication terminals.
In the radio communication system using the OFDMA scheme and the adaptive modulation, a different modulation method is applicable to each communication channel. Therefore, it is preferable that the radio base station allocates a communication channel having fine communication quality to the radio communication terminal out of communication channels which are allocatable to the radio communication terminal.
Meanwhile, the following method has been proposed as a technique which enables a communication channel having fine communication quality to be allocated to a radio communication terminal (see Patent Document 1, for example). Specifically, a radio communication terminal measures communication quality of a radio signal transmitted by a radio base station for each subcarrier and notifies the radio base station of the measured communication quality. The radio base station allocates a communication channel including a subchannel having fine communication quality to the radio communication terminal.    Patent Document 1: Japanese Patent Application Publication No. 2005-151567 (pp. 18-19, FIG. 7)